According to data from NASA's Lunar Reconnaissance Orbiter (LRO), ice may make up as much as 22 percent of the surface material in Shackleton crater at the Moon's south pole.

The huge crater, named after the Antarctic explorer Ernest Shackleton, is two miles deep and more than 12 miles wide. The small tilt of the lunar spin axis means Shackleton's interior is permanently dark and very cold. Researchers have long thought that ice might collect there.

When a team of NASA and university scientists used LRO's laser altimeter to examine the floor of Shackleton crater, they found it to be brighter than the floors of other nearby craters around the South Pole. This is consistent with the presence of small amounts of reflective ice preserved by cold and darkness. The findings are published in today's edition of the journal Nature.

In addition to the possible evidence of ice, the group's map of Shackleton revealed a remarkably preserved crater that has remained relatively unscathed since its formation more than three billion years ago. The crater's floor is itself pocked with several small craters, which may have formed as part of the collision that created Shackleton.

"The crater's interior is extremely rugged," said Maria Zuber, the team's lead investigator from the Massachusetts Institute of Technology in Cambridge in Mass. "It would not be easy to crawl around in there."

While the crater's floor was relatively bright, Zuber and her colleagues observed that its walls were even brighter. The finding was at first puzzling. Scientists had thought that if ice were anywhere in a crater, it would be on the floor, where no direct sunlight penetrates. The upper walls of Shackleton crater are occasionally illuminated, which could evaporate any ice that accumulates.

"The brightness measurements have been puzzling us since two summers ago," said Gregory Neumann of NASA's Goddard Space Flight Center in Greenbelt, Md., a co-author on the paper.

A theory offered by the team to explain the puzzle is that "moonquakes"-- seismic shaking brought on by meteorite impacts or gravitational tides from Earth -- may have caused Shackleton's walls to slough off older, darker soil, revealing newer, brighter soil underneath. Zuber's team's ultra-high-resolution map provides strong evidence for ice on both the crater's floor and walls.

"There may be multiple explanations for the observed brightness throughout the crater," said Zuber. "For example, newer material may be exposed along its walls, while ice may be mixed in with its floor."